This invention relates to plasma etching. More particularly, this invention relates to plasma etching of large panel etch substrates for large field emission display devices.
Thin film etching processes fall into two broad categories. One category is conventional liquid phase chemical etching or xe2x80x9cwet etchingxe2x80x9d. The other is gas phase plasma-assisted etching or xe2x80x9cdry etchingxe2x80x9d.
There are two primary types of dry etch mechanisms: (a) a physical mechanism and (b) a chemical reaction mechanism. In the physical etch mechanism, ions are extracted from a glow discharge and accelerated towards an etch structure whose surface is eroded by momentum transfer upon being hit by the ions. The etch structure typically includes a layer to be etched (xe2x80x9cetch layerxe2x80x9d), an overlying patterned layer that serves as an etch mask, and an underlying supporting substructure. In the chemical reaction etch mechanism, a glow discharge is employed to generate chemically active ions which diffuse to the etch structure where they react with the surface of the etch structure to produce volatile products.
An important factor related to the type of mechanism used in the etch process is selectivity. Selectivity is a measure of etch rates of different materials. Dry etch processes in which different materials are etched at approximately the same rate are referred to as nonselective. These processes typically use physical etch mechanisms. Dry etch processes in which different materials are etched at substantially different rates are referred to as selective. Some selective etch processes use chemical reaction mechanisms in which chemically reactive ions preferentially react with one material over another. In other selective etch processes, etched material is preferentially redeposited on one material over another.
In some dry etch processes, both types of etch mechanisms are present. In these processes, chemically active ions are extracted from a glow discharge and accelerated toward the etch structure. As a result, the surface of the etch structure is etched by momentum transfer and by chemical reaction.
Reactive ion etching (RIE) is an example of a dry etch process in which both types of etch mechanisms are present. Chemically active ions (reactive ions) are accelerated towards an etch structure which is etched by momentum transfer upon being hit by the reactive ions and by chemical reaction with the reactive ions.
A conventional RIE reactor is schematically shown in prior art FIG. 1. The RIE reactor in prior art FIG. 1 includes a reaction chamber 10 and an electrode 12 capacitively coupled to a high frequency power generator 13. An etch structure 14 is placed on electrode 12. In operation, a suitable feed gas is introduced into reaction chamber 10, and a glow discharge, shown as region 16, is formed. Since electrons are more mobile than ions, electrode 12 acquires a negative self-bias voltage. Positively charged ions are attracted to electrode 12 and etch structure 14, and reactive ion etching occurs.
A cross-sectional view of a typical etch structure in which a mask 20 overlies an etch layer 22 is shown in prior art FIG. 2a for an RIE process. Mask 20 defines apertures 24 through which etch layer 22 is etched. Prior art FIG. 2b is an expanded cross-sectional view illustrating the reactive ion etch of a single aperture 24 in prior art FIG. 2a. R+ represents reactive ions in prior art FIG. 2b. As reactive ions R+ travel through mask aperture 24, they collide with the aperture side walls and with other gas molecules. The collisions result in physical and reactive etching as well as recombination with free electrons. As shown, item 26 identifies a region of physical etching, item 28 identifies a region of reactive etching, and item 30 identifies a region of ion-electron recombination.
A deficiency in the reactive ion concentration occurs near the surface of etch layer 22 in apertures with high aspect (depth/width) ratios. Since etch rates are dependent upon reactive ion concentration, the deficiency results in a relatively low etch rate of etch layer 22. This low etch rate, in combination with the relatively high etch rate for mask 20 due to the substantial reactive ion concentration near the aperture opening, results in a loss of selectivity in the etch between etch layer 22 and mask 20 in apertures with high aspect ratios.
Recently there is a trend towards use of low-pressure high density plasmas in dry etch processes. As the name suggests, low-pressure high density plasmas are characterized by high densities of charged and excited species at low-pressures. This trend is fueled as minimum feature sizes are reduced to submicrometer dimensions, and aspect ratios increase. Horiike, xe2x80x9cIssues and future trends for advanced dry etching,xe2x80x9d ESC Conference, May 1999 (19 pages) discusses present issues and future trends of dry etching processes employing inductively coupled plasma (ICP), electron cyclotron resonance (ECR), and helicon wave technologies.
In ECR technology, microwave energy is coupled to the natural resonant frequency of electron gas in the presence of a static magnetic field. A conventional ECR waveguide apparatus is schematically shown in prior art FIG. 3. The apparatus includes a waveguide 40 that directs microwave energy 42 into a reaction chamber 50. Process gases are fed into reaction chamber 50. Reaction chamber 50 is surrounded by one or more coils 46 that produce an axial magnetic field. An etch structure 48 is located within reaction chamber 50. Intense electron acceleration is experienced in an ECR layer 52 that sustains the plasma.
In helicon wave technology, a plasma is magnetized longitudinally, and coupling is achieved by a radio frequency (RF) transverse electromagnetic helicon wave. A conventional helicon wave plasma apparatus is schematically shown in prior art FIG. 4. An antenna 60 is used to couple power into a reaction chamber 62. Reaction chamber 62 is surrounded by one or more coils 64 that produce an axial magnetic field. Electrons which resonate with the phase velocity of the helicon wave are accelerated and sustain the plasma. An etch structure 66 is located within reaction chamber 62.
In ICP technology, an inductive element is used to couple energy from an RF power source to ionize gas. A conventional ICP apparatus using a spiral coupler is schematically shown in prior art FIG. 5. The apparatus includes an inductive element 70 to which an RF power source is connected. Inductive element 70 is separated from a reaction chamber 72 by a quartz vacuum window 74. An etch structure 76 is located within reaction chamber 72. As RF current flows through inductive element 70, a time varying RF magnetic flux induces a solenoidal RF electric field within reaction chamber 72. This inductive electric field accelerates free electrons and sustains the plasma. The ICP apparatus shown in prior art FIG. 5 is also referred to as a transformer coupled plasma (TCP) apparatus.
Bassiere et al, PCT Patent Publication WO 94/28569, discloses a method of manufacturing microtips display devices using heavy ion lithography. The method uses a mask that typically consists of polycarbonate for etching a metal gate layer. Bassiere et al cites RIE as an example of an etch process which can be used to etch the gate layer metal. However, use of RIE to etch high aspect ratio apertures inevitably results in substantial degradation of the mask layer due to the dominant nonselective physical etch mechanism. It is desirable to have a process for selectively etching an etch layer, and in particular a metal gate layer, using a polycarbonate mask without significantly eroding the polycarbonate mask.
In accordance with the present invention, a method is provided for etching an etch layer using a polycarbonate layer as an etch mask. The etch method of the present invention may be used to fabricate gated electron emitters for large size emissive devices. The etch structure of the invention typically comprises a large glass panel of about 320 mm by 340 mm. The etch method of the present invention eliminates prior etch methods of baking the etch substrate prior to etching the substrate.
The method includes placing an etch structure in a reaction chamber. The etch structure includes an etch layer underlying a polycarbonate layer through which apertures extend. The etch layer is then etched through the apertures using a low-pressure high density plasma. The low-pressure high density plasma is generated at a pressure of approximately 1 to 30 millitorr, preferably 1 to 20 millitorr, with the ionized particle concentration being at least 1011 ions/cm3. Also, the ionized particle concentration is substantially the same throughout the entire volume of the reaction chamber.
At low-pressure, the mean free path (MFP) of reactive ions increases. This increases the reactive ion concentration at the etch layer which, in turn, increases etch selectivity. By using the etch method of the invention, nearly 100% etch selectivity between the etch layer and the polycarbonate layer is obtained.
In one embodiment, the etch layer is formed with chromium. In this embodiment, a process gas is ionized to produce a low-pressure high density plasma that contains chemically active oxygen-containing ions and chemically active chlorine-containing ions.
The etch layer can alternatively be formed with aluminum. In this embodiment, a process gas is ionized to produce a low-pressure high density plasma that contains a combination of chemically active oxygen-containing ions, chlorine-containing ions and bromine-containing ions.
The etch layer can also be formed with molybdenum. In this embodiment, a process gas is ionized to produce a low-pressure high density plasma that contains a combination of chemically active fluorine-containing ions, nitrogen-containing ions, hydrogen-containing ions and oxygen-containing ions.
The etch layer can also be formed with tantalum. In this embodiment, a process gas is ionized to produce a low-pressure high density plasma that contains a combination of chemically active fluorine-containing ions and chlorine-containing ions.
Furthermore, the etch process of the present invention can be used to etch an electrically insulating etch layer such as a silicon nitride, silicon oxide, or glass layer. The electrically insulating etch layer is etched by ionizing a process gas to produce a low-pressure high density plasma which contains chemically active fluorine-containing ions.
The etch method can be performed in a transformer coupled plasma apparatus in which the low-pressure high density plasma is generated by coupling RF power to a remote induction coil. Alternatively, the etch method can be performed in an electron cyclotron resonance apparatus or in a helicon wave apparatus.
The etch structure is cooled from room temperature of about 27xc2x0 C. to increase the etch rate. The etch structure is typically cooled to a temperature of at least approximately 5xc2x0 C. Further, the etch structure is heated to a temperature no higher than 5xc2x0 C. below the glass transition temperature of the polycarbonate layer.
The etch structure can be biased to attract reactive ions to the etch structure. This increases the concentration of reactive ions at the etch layer, and hence increases the etch rate.
An inert gas can be added to the process gas mixture. The inert gas decreases the reactive ion concentration at the etch layer, and hence decreases the etch rate.
The etch method of the invention can be used to fabricate gated electron emitters on large substrates. In one example, an electrically insulating layer underlies an electrically non-insulating (ENI) etch layer, and a second ENI layer underlies the electrically insulating layer. As used here, ENI generally means electrically conductive or electrically resistive. The ENI etch layer is then etched in accordance with the present invention. Subsequent to the etch, dielectric open spaces are created in the electrically insulating layer. Electron-emissive elements are then formed in the dielectric open spaces in such a way that each electron-emissive element contacts the second ENI layer.
The etch method of the present invention is adaptable to large surface etch substrates for fabricating large size emissive display devices. The etch method of the invention produces hole uniformity of hole sizes of 12 xcexcm when apertures created in the etch structure are etched. Uniformity in the present invention is achieved by increasing the process gas flow into the evacuation chamber and implementing a gas distribution of simultaneous center and edge distribution of the process gas over the entire surface of the etch structure.